Chapter 3 Configuration Of Gre; Brief Introduction To Gre - 3Com 3C13636 Configuration Manual

3Com Router 3000 Ethernet Family
Configuration Guide

Chapter 3 Configuration of GRE

3.1 Brief Introduction to GRE

I. GRE overview
Generic Routing Encapsulation protocol (GRE) can encapsulate datagrams of some
network layer protocols (e.g. IP and IPX) and allow these encapsulated datagrams to
be transferred in another network layer protocol (e.g. IP). GRE is a layer 3 tunnel
protocol of VPN, adopting a technique called Tunnel between protocol layers. Each
tunnel is a virtual point-to-point connection and can be regarded as a virtual interface
only supporting point-to-point connection in actual situation. The interface provides a
tunnel where encapsulated datagrams can be transmitted. And it can also encapsulate
and de-encapsulate datagrams at both ends of the tunnel.
To move in a tunnel, a packet must undergo the processes of encapsulation and
decapsulation, which are illustrated in Figure 3-1:
Novell IPX
Protocol
Group1
Figure 3-1 IPX network interconnection through GRE tunnel
1) The process of encapsulation
After receiving an IPX packet, the interface connected to Novell group1 first sends it to
IPX for processing. IPX decides how to route it by examining the destination address
field in its IPX header. If IPX finds that the packet should pass the network 1f (virtual
network number of the tunnel) in order to reach the destination, it delivers the packet to
the tunnel interface with the network number of 1f. After receiving the packet, the tunnel
interface performs GRE encapsulation before forwarding it to the IP module for
processing. After the IP header is encapsulated, the packet will be forwarded to the
appropriate network interface according to its destination address and the routing table.
2) The process of decapsulation
The process of decapsulation is contrary to that of encapsulation. The system
examines the destination address of each IP packet received from the tunnel interface;
if it is this router, the system removes the IP header of the packet and sends it to the
GRE module for processing (verifying key, checksum, and serial number of the packet,
etc.). After completing all the works, the GRE module removes the GRE header of the
packet and sends it to the IPX module where it is handled just as a common one.
Internet
Tunnel
Router A
3Com Corporation
3-1
Chapter 3 Configuration of GRE
Novell IPX
Protocol
Group2
Router B